Internal combustion engines typically drive several accessories through a belt arrangement operating off the engine crankshaft. Eor example, an exemplary vehicle engine might drive such accessories as: an air pump, an alternator, a fan, an air conditioning compressor, a power steering pump and a water pump. A well recognized problem with such accessories as conventionally belt driven is that their rate of rotation is proportional to engine speed. Consequently, the designs of the accessories have been subject to substantial compromise in order to insure adequate performance at low engine speeds and dependability and reasonable longevity at high engine speeds. In recognition of these contradictory requirements, it has been proposed in the past that a variable speed accessory drive be employed which would serve to drive the accessories relatively faster at lower engine speeds and relatively slower at higher engine speeds.
If such an effective variable speed accessory drive can be realized, numerous benefits would be obtained. For example: the life of the accessory bearings would be greatly extended, power absorption at high speed would be greatly reduced such that more engine net power output would be available, fuel economy would be improved, better accessory performance at low engine speeds (such as idle) would be obtained, smaller accessories could be employed with resulting savings in initial cost and weight would be realized, etc.
In the variable speed accessory drives which have been proposed in the past, most have employed V-belt drives in which the driving and/or driven pulley is rendered variable in diameter by axially adjusting (under the influence of some speed responsive control system) the pulley sheave facing sections in order that the V-belt will ride at various depths in the variable diameter pulley. The fixed positions of the driving and driven pulley axes together with the fixed belt length results in the belt being tensioned and positioned radially in one of the driving or driven pulleys in an inverse fashion to the other of the driving or driven pulleys in a manner well known to the designers of such variable speed drives. The V-belts are composed of various compositions and have a trapezoidal cross section, the belt transmitting rotary motion at one speed from the engine to each accessory at another speed, the speed ratio being varied in a continuous or step-wise fashion from a minimum to a maximum as dependent on the geometry of the belt and the pulley system. It has been found that there are significant practical drawbacks to such an arrangement. The control and actuating systems are complex, expensive and do not enjoy longevity. The V-belt is compressed between smooth, conical sheave sections in the driving pulley by external axial forces acting on the sections to apply compression to the belt and friction between the sides of the belt in the sheave sections to prevent slippage In operation, a force unbalance caused by changes in the axial loading of the sheave sections causes the V-belt to change its radial positions in the driving pulley until a force balance is achieved or a limit range stop is reached.
For a large transmitted torque, the required axial forces exerted on the driving pulley sheave sections result in large compressive forces on the V-belt which requires that the belt have a substantial thickness to prevent its axial collapse or failure This increase in thickness increases the belt's centrifugal force and causes higher belt tension load. In addition, as the belt thickness increases, the pulley size must be increased due to higher stress loads at a given design minimum pulley radius. Further, the typical V-belt must continuously "pull out" from the compressive sheave load on leaving the variable diameter pulleys resulting in significant friction losses and belt fatigue which adversely affects the overall efficiency of the system and the operating life of the belt. Consequently, although variable speed pulley drives have successfully used V-belts in a wide range of applications, they have been severely limited in their power capabilities for more competitive smaller size equipment.
Thus, it is to a variable speed accessory drive which overcomes all these and other problems of the prior art systems to which my invention is directed